The present invention relates to an image scanner for reading a document optically with a CCD (Charge Coupled Device) image sensor or similar line image sensor, and processing, e.g., amplifying and digitizing the resulting image signal electrically.
It is a common practice with an image scanner to construct a CCD circuit and an image signal processing circuit independently on separate circuit boards. However, the problem with this kind of arrangement is that when the output of the signal processing circuit is faulty, there is no implementation for determining which of the CCD circuit and image signal processing circuit has failed easily and rapidly, i.e., the two circuits have to be tested one by one. A digital copier, for example, is made up of a CCD circuit, a signal processing circuit, an image processing circuit (IPU), a write processing circuit, etc. Assume that a pattern generator is built in each of the signal processing circuit and write processing circuit. Then, the write processing circuit can be tested if the output of the pattern generator built therein is written and then printed. When the write processing circuit is not faulty as determined by the test, the signal processing circuit can be tested if the output of the pattern generator built therein is processed and then applied to the write processing circuit to be printed thereby. Hence, when the reproduction of a document is defective despite that the write processing circuit is normal, it can be determined that the scanner is faulty. However, which of the CCD circuit and signal processing circuit included in the scanner is faulty cannot be determined. As a result, a fault cannot be located without consuming a substantial period of time.